Generally, high quality Lube base oil has a high viscosity index, good stability (to e.g. oxidation, Thermal, UV, etc.) and low volatility. A classification of the quality of lube base oil according to the API (American Petroleum Institute) is shown in Table 1 below.
TABLE 1Sulfur (%)Saturate (%)VI (Viscosity Index)Group I>0.03<9080~120Group II≦0.03≧9080~120Group III≦0.03≧90≧120Group IVAll PolyAlphaOlefins (PAOs)
Among mineral oil-based base oil products, base oil produced by solvent extraction mainly corresponds to Group I, base oil produced by hydrotreating mainly corresponds to Group II, and base oil having high viscosity index produced by high-degree hydrocracking mainly corresponds to Group III.
In the case where base oil is classified according to the viscosity grade, it may include Neutral base oil and Bright Stock base oil, in which the Neutral base oil typically comprises an oil fraction streaming from the tower upon vacuum distillation and the Bright Stock base oil comprises an oil fraction having very high viscosity streaming from the bottom of the tower upon vacuum distillation. In particular, base oil of Group III which is high quality Neutral base oil is referred to as Neutral in the sense that a base oil feedstock having high acidity is converted into a neutral material after refining.
The conventional preparation of a feedstock for producing Lube base oil using unconverted oil which is a heavy oil fraction that is not converted into fuel oil but remains in a fuel oil hydrocracking process is known to be a method of effectively manufacturing a feedstock of high quality lube base oil and fuel oil, as disclosed in Korean Examined Patent Publication No. 96-13606, in which unconverted oil (UCO) is drawn out directly during the recycle mode operation of a vacuum gas oil (VGO) hydrocracker to provide a feedstock for producing base oil, so that loads on first vacuum distillation (V1, atmospheric residue vacuum distillation) and hydrotreating and hydrocracking (R1 and R2) are reduced without the need to recycle the VGO back to the first vacuum distillation process (V1). Accordingly, a feedstock of high quality lube base oil having a viscosity such as 100N, 150N or the like may be prepared at significantly increased efficiency. In this case, however, conversion of UCO having various properties produced in a variety of hydrocrackers into high quality Lube base oil is left out of consideration. (manufacturing high quality lube base oil using UCO having various properties produced in a variety of hydrocrackers is left out of consideration)
Specifically, refineries all over the world include a large various type of hydrocrackers (e.g. low-pressure hydrocracker, high-pressure hydrocracker, single-stage hydrocracker, two-stage hydrocracker, one-through, recycle mode etc.), and the feedstock thereof is very diverse (such as vacuum gas oil (VGO) or coker gas oil (CGO) and which is also depend on crude oil species adapted for the corresponding refinery). Thus, the hydrocracked residue may be produced in a large variety of different ways depending on the type and operating condition of hydrocracker and its feedstock, so some may be appropriate for high quality lube base oil production and some may be inappropriate for lube base oil production. For example, there may be hydrocracked residue favorable in terms of yield, hydrocracked residue favorable in terms of properties (including viscosity index, impurity content, etc.) of lube base oil products, or hydrocracked residue unfavorable or favorable in terms of both yield and properties. In this way, hydrocracked residue species produced using various crude oil sources, various hydrocracking feedstocks (VGO or CGO), or various type of hydrocrackers (single-stage, two-stage, high-pressure (P>about 150 kg/cm2g), low-pressure (P=about 100 kg/cm2g) hydrocrackers, one-through, recycle mode etc.) may have diverse properties. Furthermore, as the size of plants that produce lube base oil has recently increased, a large amount of feedstock such as hydrocracked residue (i.e. UCO) is required to perform catalyst dewaxing and hydrofinishing, but it is very difficult to produce it in a single hydrocracker. Hence, there is an urgent need for methods that effectively and economically utilize UCO having various properties from a variety of different sources.
Also, in order to manufacture high quality base oil (Group III) having high stability at high yield using the process adapted for the properties and demands of such UCO, dewaxing and hydrofinishing reactors should be optimized. In the case of dewaxing reactors used in conventional processes that produce base oil, no consideration is given to the chimney tray for uniformly dispersing a liquid/gas mixture in catalyst beds so as to maximize the use of catalyst. Also, in a quenching zone which is provided between catalyst beds so that high-temperature gas and liquid flowing down from the catalyst beds get mixed with a quenching fluid and thus are uniformly cooled below a predetermined temperature, methods able to increase the residence time of the quenching fluid to make it as long as possible for space efficiency and unclogging purposes have not been devised.
Moreover, in the hydrofinishing process, the hydrogen partial pressure should be as high as possible in order to impart final Lube base oil products with high stability (to e.g. oxidation, Thermal, UV, etc.). However, hydrogen partial pressure is lowered due to the consumption of hydrogen during the dewaxing process, conducted before the hydrofinishing process. Therefore, methods of maintaining enough hydrogen partial pressure so that the hydrofinishing process can be performed are in demand.